I don't understand what's unintuitive here. I hit a mob for 1M. The health bar above his head goes down by 1M. What's so illogical about that?
It sounds like you want my 1M hit to be reduced to 500K (assuming two linked mobs), but then also pop up as 500K on the other mob. That seems far more illogical to me - my attack does 1M damage, unless I'm hitting a linked mob, in which case it's reduced (but, by the way, copied to the others).
The combined health bar is not invisible. It's above every linked mob. There is no combining and redividing. Every point of damage you do is a point off the shared health bar.
Cleaving is the only time it gets weird, when I hit both mobs for 1M each, but then 2M is taken off both their bars - I can see how that might look like you did 4M damage. But the key thing to realise is that each mob doesn't HAVE his own health. That's why there's no "redividing" going on, there's nothing to divide between. There's only one health bar, the game just shows it to you multiple times